As is known, in a current assignment system, current is assigned to equipment branches (or loads) with the aid of a switch arrangement by way of switches, in particular power switches with a low voltage range. These switches are correspondingly designed for a rated current, and interrupt the current flowing through the switch in the event of a fault, for example a short circuit. At such a time, only the equipment branch associated with the fault or closest thereto is disconnected correspondingly. Such behavior is called selective disconnection.
Inside each switch is a mutual inductor device (Wandlereinrichtung) and a trigger unit. The mutual inductor device correspondingly measures the current flowing through the switch by way of a mutual inductor, wherein the trigger unit correspondingly checks whether a previously given current condition is satisfied.
In order to disconnect selectively, these switches communicate with each other. For example, when a current condition in a postpositionally placed switch (as viewed from the feed direction) has been satisfied, for instance because a short circuit has occurred, that switch informs a prepositionally placed switch (as viewed from the feed direction) of this situation by way of a signal (locking signal). As a result, the prepositionally placed switch, which similarly recognizes the short circuit, will not itself trigger immediately, but will wait for a fixed period of time (a preset delay time) to see whether the postpositionally placed switch triggers. If the postpositionally placed switch has not triggered by the time the delay time has ended, the prepositionally placed switch interrupts the current itself.
The communication path is in general correspondingly conducted in the feed direction, i.e. opposite to the direction of energy flow, from the postpositionally placed switch to the prepositionally placed switch. This form of selectivity is called reduced-time selective control ZSS (zeitverkürzte Selektivitätssteuerung), also well known as ZSI (zone selective interlocking).
A defect of this is that the current direction cannot always be clearly determined, but can vary, for instance when there are multiple feed power supplies or the load is in the form of an electric motor. For example, when there are multiple feed power supplies, even if one power supply is disconnected or connected without interference in the normal operating state, this will lead to a reversal of the energy flow direction in at least one switch. Considering the energy flow direction by way of well-known communication connections will lead to a confused jumble of numerous connections (the majority being wire connections). Since each energy flow direction correspondingly needs one input and one output, the ZSI connections (ZSI communication) of a switch need a total of four connection terminals.